A membrane carburetor of the kind referred to above is disclosed in U.S. Pat. No. 4,903,655. The valve member comprises a guide body having a valve body at one end and, at the other end thereof, the guide body is held in a bifurcated holder of a control lever. The control lever is pivotally journalled in the housing of the membrane carburetor. The other end of the control lever is, on the one hand, resiliently biased by a control spring in the direction of closure of the inlet valve and, on the other hand, is actuable by the control membrane against the force of the spring in the direction of opening the inlet valve. The valve member is guided in the stroke direction by the guide body in the feed channel. The guide body is self-supporting in the feed channel with radial play. The guide ribs run in the direction of the feed channel and are arranged over the periphery.
When the internal combustion engine draws combustion air in through the venturi section, fuel leaves through the idle nozzle when the throttle flap is closed whereby an underpressure develops in the control chamber. The control membrane moves into the control chamber and acts on the control lever in the opening direction of the inlet valve. The valve body then lifts away from the valve seat. Fuel then flows into the control chamber to equalize the pressure. After the pressure equalization has taken place, the control membrane moves back into its start position and the inlet valve is closed by the action of the control spring. This alternating action ensures that the control chamber is filled with fuel having a pressure level in the vicinity of atmospheric pressure.
The vibrations of the internal combustion engine which occur at idle also act on the carburetor even when this is mounted so as to be decoupled from the engine. These vibrations impart corresponding acceleration forces to the valve member and these vibrations can lead to an unwanted opening of the inlet valve whereby too much fuel enters into the control chamber which is then supplied in an uncontrolled manner to the venturi section via the openings so that the mixture becomes enriched. It is especially at idle that the uncontrolled fuel inflow (main nozzle drip) leads to enrichment of the mixture and therefore to fluctuations in the idle engine speed such as a drop in engine speed and, in the extreme case, causing the engine to die because of overenrichment. The engine must then be started anew.
Based on theoretical considerations, acceleration forces act because of the vibrations on the valve member transversely to the opening direction and in the opening direction. The acceleration forces acting in the opening direction can be compensated by an appropriately dimensioned control spring. Acceleration forces occurring transversely to the opening direction cannot be transmitted directly to the inner wall of the feed channel since the guide body is guided in this channel with radial play. For this reason, acceleration forces acting transversely to the opening direction lead to a radial displacement of the valve member so that the valve cone is pressed against the valve seat transversely to the opening direction. The acceleration force is then distributed in accordance with a vector diagram and a further force results acting in the opening direction. To compensate for this force, the control spring must be correspondingly stronger dimensioned. A control spring which is dimensioned too strong however influences the formation of the mixture and therefore the operating performance of the engine since a higher underpressure must then be present in the venturi section for opening the inlet valve to the control chamber and this higher underpressure must be developed by the engine.
On the other hand, the radial play of the guide body in the inlet channel can be dimensioned smaller in order to obtain a better bracing of the transverse forces on the inner wall of the inlet channel. These theoretical considerations have been substantiated in laboratory operation; however, in practice, it has been determined that the slight radial play leads to an early freezing of the guide body in the feed channel since dirt particles are always conveyed with the fuel. For example, with tank venting, the finest dust can penetrate which over time causes the guide body to become seized and renders the membrane carburetor inoperable. Accordingly, to prevent seizure of the valve member, a specific radial play may not be reduced below a specific value.
In modern motor-driven chain saws, the carburetor is mounted separately from the engine in the housing of the chain saw such as in the handle because of thermal considerations. The carburetor then is connected to the engine via elastic channels. Decoupled carburetors of this type are greatly subjected to different vibrations depending upon peripheral conditions. The carburetor of a motor-driven chain saw is set in the test stand after manufacture. The idle engine speed is stable below the coupling speed of the centrifugal clutch which drives the saw chain. In practice, this setting has proven successful when the motor-driven chain saw is held in the hand. However, if the operator sets the motor-driven chain saw down for example on a concrete surface, then vibrations having an increased amplitude occur in the carburetor which impart corresponding accelerating forces on the valve member whereby the inlet valve opens in an uncontrolled manner. Main nozzle drip occurs and the idle engine speed changes greatly or the machine dies. When the motor-driven chain saw is set down on the forest ground, then different vibration and force relationships occur. A setting of the membrane carburetor to a constant idle engine speed which is influenced only slightly by occurring vibrations is hardly possible.